The hotspot, detected first by a local resident a week ago, had reached 480 microsieverts per hour. The ward said in a news release that it found the container buried at the park and was able to dig it out with help from experts. Radiation levels have dropped to normal levels after workers removed soil from an area of about 30 centimeters in diameter and 10 centimeters deep.

Radium was once used for medical and other purposes, including for glow-in-the-dark paint on antique watches. The ward said it wasn't clear how the container ended up buried at the site. The park opened in 2013 on land where a garage for garbage and cleaning trucks operated by the Tokyo metropolitan government once stood.

The ward said Saturday that the park is safe since the container was sealed and the grounds were not contaminated. It also said that the radium is unlikely to have caused any health issues, because a person would have had to stand right over the hotspot for more than 200 hours to be exposed to a radiation level shown to increase cancer risk.

Toshima ward says any area with radiation levels above 0.23 microsieverts per hour must undergo decontamination in line with an environment ministry hourly limit. The ministry’s annual limit is 1 millisievert, or 1,000 microsieverts.